Magneto ignition systems of the type described in the referenced U.S. Pat. No. 4,188,929 have been found eminently suitable for small internal combustion (IC) engines, particularly in those in which space is at a premium. In many such engines, there is insufficient room for special ignition timing transducers. The magneto generator is thus frequently connected to a fan or ventilator wheel on the IC engine, by embedding permanent magnets in the rotating fan wheel, formed with two pole shoes, which are magnetically coupled to an armature secured to the housing of the IC engine. As described in the referenced patent, the armature is wound on a core which carries both a primary and secondary winding, the secondary winding being connected to a spark plug, and the primary winding being connected to a control network which includes an ignition transistor. The ignition transistor is controlled to conduction or blocking state by a control circuit which includes a control transistor.
The ignition transistor which, preferably, is a Darlington transistor, is controlled to block by the control transistor at the ignition instant. The control transistor has its base connected to a circuit which includes a Zener diode, as well as two R/C networks which are timing networks to control the ignition instant. A first series R/C circuit generally determines the ignition instant, and a second series R/C circuit, connected in parallel thereto, is provided to control change of the ignition instant with change in speed of the engine, typically by advancing the ignition at high speed.
It has been found that this system, which is eminently suitable for small engines, under some operating conditions, not reliably prevent reverse operation of the engine. This, however, is desirable for safety reasons if an ignition system with such an engine is coupled to a utilization device which should operate in only one direction, for example a chain saw or a lawn mower. Engines driving such appliances should reliably prevent reverse direction operation within a relatively wide range of speed.